Optical power limiter

ABSTRACT

An optical power limiter structure comprises an asymmetric waveguide showing a defocussing nonlinearity of the core region material. The structure comprises a cladding region layer (2) and a core region layer (3) grown on a substrate (1). The core region layer (3) upper surface is exposed to air. By using an optical signal of wavelength λ slightly greater than the bandgap equivalent wavelength of the core region material, the structure exploits a defocussing nonlinearity of semiconductor materials in a manner enhanced by the asymmetry of the waveguide.

The present invention relates to optical power limiters. It findsparticular application in optical communications.

A device which has considerable potential for use in opticalcommunications systems is an optical limiter. Placed prior to thephotodetector in an optical receiver, it could alleviate problemsencountered with dynamic range limitations in conventional receivers.

Different forms of optical limiter have been proposed and demonstrated,a number of which make use of nonlinear optics. If the limiter is formedin an optical waveguide then there are several additional advantages,foremost amongst these being the ability to confine the radiation tosmall dimensions, ie high power densities, and thus achieve limitingaction at relatively modest powers by comparison with bulk(non-waveguide) limiters. Furthermore, if the waveguide is made of asemiconductor material then there exists the potential for integrationof the limiter with the photodetector and with other optical componentsalso fabricated as semiconductors.

(An optical waveguide comprises a core region of a first material, andan outer, cladding region of a second material. The refractive index ofthe core material is higher than that of the cladding material, havingthe effect of guiding optical radiation transmitted by the waveguide topropagate at least substantially along the core region. Neither the corenor the cladding region necessarily exhibits only one uniform refractiveindex. In epitaxially grown semiconductor waveguides the core generallyhas a rectangular cross section, the cladding being provided bycharacteristics of the layers of the semiconductor structure concerned).

Waveguide optical limiters are known which rely on nonlinear opticalbehaviour resulting in changes in refractive index with input signalintensity. They rely on the characteristics of certain materials thatrefractive index increases with optical intensity. Hence they can bedesigned so that the difference in refractive index of core and claddingmaterials reduces as input signal intensity increases. As the differencedecreases, the waveguiding properties of the structure decrease untilwhen the refractive index difference disappears altogether, opticalradiation is no longer confined even substantially to the core region.If the output of the structure is taken from the core regions, thisshows a power limited characteristic.

Waveguide optical limiters have been reviewed by Seaton et al (OpticalEngineering, Volume 24, No. 4, pp 593-599, 1985). However, because todate most of these have relied on nonlinear optical behaviour whereinrefractive index increases with optical intensity, they have relied onnonlinearity of the cladding material of the waveguide, rather than ofthe core material. That is, as the optical intensity increases, therefractive index of the cladding material (n_(s)) increases, so reducingthe refractive index difference between core and cladding (n_(c)--n_(s)) and consequently reducing the strength of the waveguidingcharacteristic.

Although such a device can act as an optical limiter, its responseoccurs at relatively high values of input signal intensity for the fieldof optical communications. This is because the major part of the opticalradiation is carried by the core material, not the cladding material,but it is the response of the cladding material which must produce thelimiting effect.

Some analyses have been published pertaining to waveguide structureswhere the nonlinear behaviour occurs in the core (eg Boardman and Egan,IEEE Journal of Quantum Electronics, Vol. QE-22, No. 2, pp 319-3241986), but because again they rely on materials whose refractive indexincrease with intensity, the situation considered is that of aself-focussing nonlinearity, ie one where the refractive index of thecore material increases with increasing optical intensity, rather thanthe opposite case of a defocussing nonlinearity. There is therefore noobvious way to make an optical limiter with such a structure.

There is a publication by Ogusu (Optical and Quantum Electronics, Volume19, No. 2, pp. 65-72 1987) which considers a waveguide with adefocussing nonlinear characteristic. Ogusu also states that thewaveguide can be used as an optical power limiter. However, the opticalintensities at which Ogusu's structure would limit the power are likelyto be far higher than those of interest in optical communicationssystems. If the waveguide structure disclosed were designed to limit atlower intensities, then the tolerances placed on refractive indexdifferences between core and cladding materials would be so fine as topreclude the device ever being realised in a practical embodiment.Further, the waveguide does not comprise semiconductor materials, andrelies on optical characteristics not available in practicalsemiconductor structures. It cannot therefore even potentially beintegrated with a photodetector or other optical component.

It has been realised, in making the present invention, that it ispossible to exploit characteristics of semiconductor materials so as toproduce a practical device with an optical power limiting action,suitable for use at the powers relevant to optical communicationssystems.

According to the present invention, there is provided an optical powerlimiter structure for use in limiting the power of an optical signalhaving a wavelength λ, comprising a direct-gap semiconductor materialwaveguide, having a core region of material of refractive index n_(c)bounded on a first side by a cladding region of refractive index n_(s)and bounded on a second, opposing side by a cladding region of materialof refractive index n_(a), wherein λ is slightly greater than theband-gap equivalent wavelength of the core region material, λ_(c), butsubstantially greater than the band gap equivalent wavelength of thecladding region bounding the .[.second.]. .Iadd.first .Iaddend.side,λ_(s), and wherein n_(c) varies inversely with the intensity of theoptical signal at the wavelength λ, but n_(s) and n_(a) remain at leastsubstantially constant, such that at a preselected threshold intensityof the optical signal, n_(c) becomes at least substantially equal ton_(s).

By selecting λ to lie slightly above λ_(c), it is possible to exploitthe largest optical nonlinearity occurring in semiconductor materials;that associated with resonant absorption and refraction near the bandedge. For input signal photon energies just below the band edge of thecore material it is possible to access a fairly strong nonlinearity inthe refractive index, whilst the material in the cladding layers can beselected to behave linearly (as long as the photon energy is too farbelow the corresponding band edge). Hence n_(s) and n_(a) remainconstant.

Semiconductor materials generally used in optical communications, forinstance those based on the elements Ga, In, As, Al and P show therequired defocussing non-linearity just below the band edge. A generalformula which can be used to predict the extent to which the refractiveindex of a semiconductor material will change with optical intensity isgiven in the publication Physical Review Letters, Volume 47, Number 3,pp 197-200, 10 Jul. 1981, by D. A. B. Miller et al.

Taking the band edge as the energy level at which absorption becomessignificant, that is at which absorption reaches 10⁴ cm⁻¹, then λ shouldbe greater than λ_(c) by an amount such that the absorption is greatenough to change carrier concentration but not great enough to decreasethe amount of light transmitted by a limiter structure toosignificantly. In general, λ should be greater than λ_(c) by a value inthe range from 10 to 40 nm inclusive. This effectively selects the mostsignificant range of the optical nonlinearity.

However λ should be greater than λ_(s) by an amount great enough toavoid significant changes in carrier concentration, and hence inrefractive index n_(s), of the cladding material bounding the first sideof the core region. Again the publication of D. A. B. Miller et alreferenced above will enable the extent to which refractive indexchanges occur for particular materials and input signals to becalculated. However in general, λ should clearly be greater than λ_(s)by at least 40 nm. Preferably it should be greater than λ_(s) by atleast 60 nm and more preferably by at least 80 nm.

By selecting an asymmetric arrangement, n_(s) being greater than n_(a),the optical limiting effect is enhanced because "trail-off" of theoptical signal into the cladding material is more exaggerated thegreater the degree of asymmetry. Hence the optical limiting effect canbe obtained at lower input signal intensities.

Preferably n_(s) is greater than n_(a) by an amount of the order of 1 ormore. More preferably, the cladding material bounding the second side ofthe core region is air since this gives a semiconductor structure whichis relatively easy to fabricate by epitaxial growth techniques such asmetal organic vapour phase epitaxy (MOVPE).

The design of the waveguide is preferably such as to produce singletransverse mode operation at the operating wavelength since this isconvenient for coupling light into and out of the waveguide. Hence, ifthe waveguide comprises a substrate with a cladding region layer and acore region layer on the substrate, the opposing face of the core regionlayer being exposed to air, as a rough guide the thickness "d" of thecore region layer will be selected to be of the order given by therelationship

    2d(n.sub.o.sup.2 -n.sub.s.sup.2).sup.1/2 ≦λ  (E1)

where n_(o) =the refractive index of the core material in the adsence ofan input signal.

More accurately, in an asymmetric waveguide the thickness "d" will beselected according to the relationship ##EQU1##

Another constraint on the refractive indexes n_(o) and n_(s) is thatthey are sufficiently close for n_(c) to approach n_(s) at an inputsignal intensity which is reasonable in the circumstances in which theoptical power limiter structure will be applied.

To make an optical power limiter from an optical power limiter structureaccording to an embodiment of the invention, it will generally benecessary to apply means for selecting an output signal. For instancethis might comprise a combination of means for picking up the signalemitted from the core region of the waveguide and means for effectivelydiscarding radiation which has "spread" into the cladding region.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 shows schematically a cross section of an optical power limiterstructure according to a first embodiment of the present invention;

FIG. 2 shows in graph form the optical input signal distribution in thestructure of FIG. 1 (turned through 90°) plotted for difference valuesof input signal intensity;

FIGS. 3 and 4 (.Iadd.a) (b) (c) .Iaddend.show schematically crosssections of optical power limiter structures according to second andfurther embodiments of the present invention;

FIG. 5 shows an arrangement for effectively discarding radiation whichhas "spread" into the cladding region of an optical power limiterstructure as shown in FIG. 1; FIG. 5a shows a cross section of part ofsuch an arrangement; and FIG. 5b shows a plan view of such anarrangement;

FIG. 6 shows schematically an optical power limiter incorporating anoptical power limiter structure according to an embodiment of thepresent invention;

FIG. 7 shows a schematically a cross section through a nonlinear ribwaveguide;

FIG. 8 shows a multilayer division of the rib waveguide of FIG. 7;

FIGS. 9 and 10 are graphs showing results of calculations of confinementfactor versus input power for a rib waveguide device according to theinvention; and

FIG. 11 is a schematic cross section through a multiple quantum wellwaveguide device according to the invention.

Referring to FIG. 1, an optical power limiter structure comprises asemiconductor substrate 1, having a cladding region layer 2 and a coreregion layer 3 thereon. The upper surface of the core region layer 3 isexposed to air. (Terms such as "upper" which imply a specificorientation are used herein for convenience of description only andshould not be taken to indicate a constraint).

The core and cladding region layers 3, 2 are both multiple quantum well(MQW) layers. This is not an essential feature of the core and claddingregion layers 3, 2 as it has been found that, at operating wavelengthsclose to the effective band edges, MQW material and bulk samples showroughly the same nonlinear refractive index behaviour. In fact, it is animportant feature of the present invention that the nonlinear refractiveindex is a strong function of operating wavelength close to the bandedge, and it is possible to get large nonlinearities in any givensemiconductor provided low-loss transmission can be obtainedsufficiently close to the band-gap. There is however a trade-off betweenattenuation and nonlinear refractive index, and the present structurewill be subject to the effects of this trade-off just like any other.The advantage of using MQW layers is the control then available overrefractive index (in the absence of an input signal).

The MQW layers comprise the ternary material Ga In As separated bybarrier layers of Al In As, the compositions of these alloys beingchosen to lattice match InP which is the material of the substrate 1.The refractive indexes n_(o) of the core region layer 3 in the absenceof an input signal, and n_(s) of the cladding layer 2, are determined bythe thicknesses of the Ga In As layers (wells) and Al in As layers(barrier layers) of the MQW layers. The barrier layers in each case are30 Angstroms deep. In the core region layer 3, 120 Angstrom wells give arefractive index n_(o) of 3.43 at a bandgap of about 0.8 eV (equivalentwavelength 1.55 μm). In the cladding region layer 2, 60 Angstrom wellsgive a refractive index n_(s) of about 3.38 at a bandgap of 0.87 eV(equivalent wavelength 1.43 μm). The core region layer 3 is 1 μm thick,this giving single transverse mode operation at operating wavelengthsequivalent to photon energies just below 0.8 eV. The cladding layer 2 isat least 10 μm thick. The cladding layer thickness can be reduced to amore acceptable thickness if it has a lower refractive index. (The lowercladding should be thick compared with the extend of the guided modeinto it. If it is not and if it is bounded by a layer of lowerrefractive index, as is the case with Ga In As P and related compoundsin InP, the small index step between the core and the cladding behavesas merely a perturbation in a thick guide formed of the core plus thecladding, and there is very little change with increasing opticalintensity. If a substrate of suitable refractive index cannot be found,a thick (≃10 μm) epitaxial cladding layer, probably of semiconductoralloy, must be grown).

In use, a continuous wave (cw) optical input signal is fed to the coreregion layer 3, having a photon energy of about 30 meV below 0.8 eV.This will affect the effective refractive index of the core region layer3. The effective refractive index n_(c) of the core region layer 3 canbe represented as (n_(o) -n₂ I) where n₂ is the change in refractiveindex of the core region layer 3 per unit of intensity (in W cm⁻²)I ofthe input signal. At wavelengths close to the bandgap of the core regionlayer 3, n₂ is of the order of 10⁻⁵ per unit of intensity. Taking noaccount of the asymmetry of the structure regarding n_(s) and n_(a), thestructure described will start to show a power limiting characteristicwhen the input signal has an optical intensity of the order of 5×10³W/cm². That is, the equation

    n.sub.o -n.sub.2 I=n.sub.s                                 (E 3)

is true at that order of intensity. Hence the waveguiding action of thestructure would then be lost and the input signal will spread into thecladding region layer 2.

Taking account of the asymmetry of the structure however, the powerlimiting characteristic will take effect at a lower input signalintensity. In this particular case, the actual relationship ##EQU2##applies. This gives a power limiting characteristic at input signalintensities of about 3.5×10³ or less.

Equation (E4) given above is specific to the structure described abovewith reference to FIG. 1. The value of the factor 5 in the expressionλ/5d is determined by the degree of asymmetry in the structure, that is,by the relationship between n_(o), n_(a), and n_(s). In practice, itwill be found that if the asymmetry is increased, the factor 5 will bereplaced by a value approaching or equal to 4. If the asymmetry isdecreased, the factor 5 will be replaced by a greater value up to theworst case limit in which the expression λ/5d is replaced by zero. Inthat case, equation (E3) applies, the waveguide being symmetric in termsof refractive indexes n_(a) and n_(s).

For an optical spot size of the order 1 μm² the corresponding criticalpower level is about 0.035 mW, a level which is of appropriate magnitudefor avoidance of saturation effects in optical receivers incommunications.

Although in the arrangement discussed above a cw signal is used, it isnot necessary. However, it is important that fluctuations in temperatureshould not override the changes in refractive index which an opticallimiter structure according to an embodiment of the present inventionrelies on. Hence either a high repetition rate input signal should beused, or good heat sinking. Temperature fluctuations over periods of theorder of milliseconds, and possibly microseconds, may well be found todominate undesirably over the electronic effects relied on.

A theoretical nonlinear waveguide analysis of the operation of thestructure described above with reference to FIG. 1 can be used to plottransverse optical intensity distributions in the structure for variousvalues of the parameter n₂ I_(s), where I_(s) is the optical intensityvalue at the core/cladding interface. Referring to FIG. 2, it can beseen that as this parameter n₂ I_(s) increases, the proportion of theoptical input signal travelling in the cladding 5 increases, and for thevalue n₂ I_(s) =0.036, there is dramatic spreading of the opticalintensity into the cladding. Hence, at an optical intensity at thecore/cladding interface 6 of 3.6×10³ W/cm² there appears a strong powerlimiting characteristics. (It should be noted that the theoreticalanalysis used here neglects attenuation of the input signal in thestructure. It is preferably anyway that the core region at least of thestructure should show low attenuation so that the signal output of thestructure is reasonably high for a structure length which is physicallyconvenient to handle. Also, significant attenuation would producenon-uniformity of the defocussing non-linearity along the length of thestructure, which may be inconvenient.)

In another embodiment of the present invention, confinement of thesignal in the waveguide in the lateral direction may be provided as wellas in the transverse direction. For instance, referring to FIG. 3, a rib4 may be made in the top (core region) layer 3 to confine the lightlaterally. The width and height of the rib 4 must be chosen to ensuresingle-mode propagation both laterally and transversely. Once again therefractive indexes will be chosen to permit loss of waveguide action ata critical value of optical intensity. It is possible that breakdown ofwaveguiding in this structure might be arranged to occur in the lateraldirection before it occurs in the transverse direction.

An analysis has been carried out of a rib waveguide whose cross-sectionis shown in FIG. 7. The specific structure modelled corresponds to usingInGaAs/InAlAs MQW material of two different well/barrier thickness forthe nonlinear core layer and the lower cladding. However, thequantum-confinement properties of MQW material are not used in thedevice operation, and the analysis is sufficiently general to be appliedto rib guides made from other materials, provided that the refractiveindex in the core layer decreases with increasing optical intensity, andthe indices in the cladding layers are independent of optical intensity.The upper cladding is assumed to be air, and the operating wavelength istaken as 1.55 μm.

The method of analysis involves a multilayer division of the structurein the direction along the epilayers, as illustrated in FIG. 8. Atpresent the results obtained are somewhat sensitive to the number ofdivisions taken, and the optimum number for accuracy in the limit whenthe optical field spreads a significant distance has not yet beenfinally determined.

In its proposed application as an optical power limiter, the deviceoperates by loss of optical confinement along the plane of the epilayers(the x-direction). Light launched into one end of the rib structure, isthen not guided to the other end in the usual fashion, but may propagatefreely in the slab. A modified version of the device could use absorbingmaterial in the slab to eliminate this optical power or, alternatively,a second rib could be used to collect the power and channel it elsewhere(in the latter case, the device begins to resemble a nonlineardirectional coupler).

The calculations have concentrated on the behaviour of the opticalconfinement in the x-direction. FIG. 9 shows results calculated for arib of width w=3 μm, total height t=1.3 μm, and a slab of thickness d=1μm, these values being chosen to correspond to a single-mode guide atthe cut-off of the first higher-order mode for low optical intensities.The curves labelled I, II and III correspond, respectively, to taking 1, 3 and 45 divisions of the structure as shown in FIG. 8. In FIG. 9 theconfinement factor is plotted versus a normalized power which includesthe strength of the optical nonlinearity as a parameter. In order toexpress the results in terms of real power, it is necessary to assume avalue for the optical nonlinearity coefficient. This is done in FIG. 10,where it has been assumed that the refractive index decreases by 10⁻⁵for every W/cm² of optical intensity. In FIG. 10, the two curvescorrespond to widths w of 3 μm (for which the height t is 1.3 μm) and 5μm (for which the value of t is 1.1 μm to ensure single-mode operation);in each case the slab thickness d is 1 μm. The calculations arediscontinued before the sensitivity to the number of divisions usedbecomes too critical. From the results it is clear that a substantialloss of optical confinement is achieved for launch powers of order a few10's of μW.

It is important to note that the ribe structure as shown in FIG. 3 isonly one possible embodiment of a two-dimensional structure. Referringto FIG. 4, other possible versions are the raised strip (ridge) guide(a), the embedded strip (buried) guide, (b), and the strip-loaded guide(c). All of these can be made to include a defocussing nonlinearity inthe core region, and may be expected to show limiting action in asimilar manner to that described for the rib guide of FIG. 3.

In achieving an effective optical power limiter using the structuredescribed above with reference to FIGS. 1, 3 and 4, it may not besufficient merely to allow the optical intensity to spread into thesubstrate, since a considerable proportion of it may still reach adetector positioned to pick up an output signal from the core region. Itmay then be necessary to include some means of removing the power fromthe cladding region layer 2, or at least of preventing it from reachingthe detector. One such means is to incorporate impurities in thematerial of the cladding region layer 2 to absorb the light whichenters, and thus ensure that it cannot reach the detector.

Referring to FIG. 5a, in a structure showing lateral confinement asshown in FIG. 3, another, alternative means is to include a parallelguiding layer 13, of refractive index close to, or greater than, n_(o)in order that the light spreading into the cladding region 2 is thencaptured into this second guiding layer 13. Referring to FIG. 5b, thissecond guiding layer 13 and the core region layer 3 could then bearranged to separate in a lateral direction in such a way that the lightin the second guiding layer 13 is directed away from the detector (notshown). This second guiding layer 13 has a second, associated claddinglayer 12.

Extra layers, in addition to those described above with reference to theFigures, may be incorporated in a limiter structure according to anembodiment of the present invention. For instance, extra layers may beused to give a structure particular waveguiding characteristics.Alternatively, if a structure is integrated with other components, itmay be convenient that the structure incorporates an uppermost oxidelayer. However, such extra layers will merely form part of the core ofcladding regions, in practice.

A further embodiment of the invention makes use of the possibility withMQW materials of varying independently the band gap (and hence operatingwavelength) and refractive index of a MQW material. The lowest energyabsorption edge in a MQW material is that of the wells. The position ofthe absorption edge may be shifted to higher energy by making the wellsthinner. In the present application, the refractive index of thematerial is an average of those of the wells and the barriers, since thelayer thicknesses of the wells and barriers are much smaller than theregion to which the optical fields are confined.

An example of a power limiting structure using MQW material is shownschematically in cross section in FIG. 11, and consists of a non-linearcore material with a band edge close to and less than the operatingwavelength (in this example 1.55 μm), say 1.53 μm, and with a firstrefractive index to say 3.43, and a two part lower cladding. Closest tothe core, a linear cladding layer with a refractive index slightly lowerthan the first, say 3.38, and a wider band gap, say 1.5 μm, and then asecond cladding layer of the same refractive index as the first claddingbut with its band edge, say 1.65 μm, chosen to make it absorbing at thechosen operating wavelength. The whole could be grown on an InPsubstrate (or any other suitable material) without the cladding havingto be prohibitively thick, as the absorbing layer (the second claddinglayer) will ensure that the guided mode fields do not extend to thesubstrate interface. The first, transparent, cladding layer is necessaryto prevent excessive absorption of the guided mode at low intensity.This structure has the added benefit that at high intensity, theradiation spread into the cladding regions will be absorbed andtherefore will not be observed at the output of the device.

Referring to FIG. 6, an optical power limiter based on a structure asdescribed above with reference to FIGS. 1 to 5 or 7 to 11 inclusive willshow additional features such as an input port 7 for an optical signalto be input to the core region 3, and means such as an optical fibre 8for picking up the output signal from the core region 3. The embodimentshown in FIG. 6 is not an integrated arrangement. In such anarrangement, the optical fibre 8 might be replaced by a furtherwaveguide on a common wafer with the waveguide of the limiter structure.Also, the input port 7 might be defined in another way than by anexposed facet on the end of the limiter structure, for instance by aninterface between the limiter structure and another structure. Theoptical fibre 8 might also be replaced directly by a photodetector.

We claim:
 1. An optical power limiter structure for use in limiting thepower of an optical signal having a wavelength λ, comprising adirect-gap semiconductor material waveguide, having a core region ofrefractive index n_(c) bounded on a first side by a first claddingregion of material of refractive index n_(s) and bounded on a second,opposing side by a second cladding region of material of refractiveindex n_(a), wherein λ is slightly greater than the bandgap equivalentwavelength of the core region material, λ_(c), but substantially greaterthan the bandgap equivalent wavelength of the .[.second.]. .Iadd.first.Iaddend.cladding region, λ_(s), and wherein n_(c) varies in inverselywith the intensity of the optical signal at the wavelength λ, but n_(s)and n_(a) remain at least substantially constant, such that at apreselected threshold intensity of the optical signal, n_(c) approachesn_(s) sufficiently to allow significant spread of the optical signalinto the first cladding region.
 2. A structure according to claim 1wherein λ lies about λ_(c) by an amount in the range of 10 to 40 nminclusive.
 3. A structure according to claim 1 wherein n_(s) is greaterthan n_(a) by an amount of the order of 1 or more.
 4. A structureaccording to claim 3 wherein the cladding material bounding the secondside of the core region is air.
 5. A structure according to claim 1wherein the waveguide shows single transverse mode propoagation atwavelength λ.
 6. A structure as claimed in claim 1 wherein the firstcladding region is bounded on the side remote from the core region by athird cladding region of material having a refractive index n_(t) and abandgap equivalent wavelength λ_(t), wherein n_(t) is less than n_(c)and substantially the same as n_(s), and wherein λ_(t) is greater thanλ_(c).
 7. A structure as claimed in claim 6 wherein the core region andthe materials of the first and third cladding regions each comprise amultiple-quantum-well material.
 8. An optical power limiter comprising astructure according to claim 1 an input port for an optical signal tothe core region, and means for picking up the output signal from thecore region of the waveguide.
 9. A limiter according to claim 8 whichfurther comprises preventative means for preventing optical radiationpropagating in a cladding region of the waveguide from coupling into themeans for picking up the output signal emitted from the core region. 10.A limiter according to claim .[.10.]. .Iadd.9 .Iaddend.wherein thepreventative means comprises absorbing material in the cladding region.11. A limiter according to claim 10 wherein the preventative meanscomprises a second waveguide arranged to divert optical radiationpropagating in the cladding region away from the means for picking upthe signal emitted from the core region.
 12. An optical power limiterstructure for use in limiting the power of an optical signal having awavelength λ, comprising a direct-gap semiconductor material waveguide,having a core region of refractive index n_(c) bounded on a first sideby a first cladding region of material of refractive index n_(s) andbounded on a second, opposing side by a second cladding region ofmaterial of refractive index n_(a), wherein λ is within 20 nm of thebandgap equivalent wavelength of the core region material, λ_(c), butsubstantially greater than the bandgap equivalent wavelength of the.[.second.]. .Iadd.first .Iaddend.cladding region, λ_(s), and whereinn_(c) varies inversely with the intensity of the optical signal at thewavelength λ, but n_(s) and n_(a) remain at least substantiallyconstant, such that at a preselected threshold intensity of the opticalsignal, n_(c) approaches n_(s) sufficiently to allow significant spreadof the optical signal into the first cladding region.